Scientists build world's first anti-laser

Feb 17, 2011

In the anti-laser, incoming light waves are trapped in a cavity where they bounce back and forth until they are eventually absorbed. Their energy is dissipated as heat. Credit: Yidong Chong/Yale University

More than 50 years after the invention of the laser, scientists at Yale University have built the world's first anti-laser, in which incoming beams of light interfere with one another in such a way as to perfectly cancel each other out. The discovery could pave the way for a number of novel technologies with applications in everything from optical computing to radiology.

Conventional lasers, which were first invented in 1960, use a so-called "gain medium," usually a semiconductor like gallium arsenide, to produce a focused beam of coherent lightlight waves with the same frequency and amplitude that are in step with one another.

Last summer, Yale physicist A. Douglas Stone and his team published a study explaining the theory behind an anti-laser, demonstrating that such a device could be built using silicon, the most common semiconductor material. But it wasn't until now, after joining forces with the experimental group of his colleague Hui Cao, that the team actually built a functioning anti-laser, which they call a coherent perfect absorber (CPA).

The team, whose results appear in the Feb. 18 issue of the journal Science, focused two laser beams with a specific frequency into a cavity containing a silicon wafer that acted as a "loss medium." The wafer aligned the light waves in such a way that they became perfectly trapped, bouncing back and forth indefinitely until they were eventually absorbed and transformed into heat.

Stone believes that CPAs could one day be used as optical switches, detectors and other components in the next generation of computers, called optical computers, which will be powered by light in addition to electrons. Another application might be in radiology, where Stone said the principle of the CPA could be employed to target electromagnetic radiation to a small region within normally opaque human tissue, either for therapeutic or imaging purposes.

Theoretically, the CPA should be able to absorb 99.999 percent of the incoming light. Due to experimental limitations, the team's current CPA absorbs 99.4 percent. "But the CPA we built is just a proof of concept," Stone said. "I'm confident we will start to approach the theoretical limit as we build more sophisticated CPAs." Similarly, the team's first CPA is about one centimeter across at the moment, but Stone said that computer simulations have shown how to build one as small as six microns (about one-twentieth the width of an average human hair).

The team that built the CPA, led by Cao and another Yale physicist, Wenjie Wan, demonstrated the effect for near-infrared radiation, which is slightly "redder" than the eye can see and which is the frequency of light that the device naturally absorbs when ordinary silicon is used. But the team expects that, with some tinkering of the cavity and loss medium in future versions, the CPA will be able to absorb visible light as well as the specific infrared frequencies used in fiber optic communications.

It was while explaining the complex physics behind lasers to a visiting professor that Stone first came up with the idea of an anti-laser. When Stone suggested his colleague think about a laser working in reverse in order to help him understand how a conventional laser works, Stone began contemplating whether it was possible to actually build a laser that would work backwards, absorbing light at specific frequencies rather than emitting it.

"It went from being a useful thought experiment to having me wondering whether you could really do that," Stone said. "After some research, we found that several physicists had hinted at the concept in books and scientific papers, but no one had ever developed the idea."

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If standard laser stimulate photons to be emitted and so absorbed by the target, shouldn't this 'antilaser' (lasar?) stimulate photons to be absorbed and so emitted by the target?So if we constantly excite a sample which is surrounded by detectors in all directions but the one to the antilaser, shouldn't turning it on cause that detectors caught smaller amount of light ... earlier?I thought about something similar, but conceptually simpler - imagine CPT transformation of free electron laser:thescienceforum.com/viewtopic.php?p=255510

I remember something I read in school about a theroritical 'black hole' so to speak. It was a cavity shaped like an inverted vase coated with a black substance on the inside so that no light shined upon it ever came out. There were infinite reflections on the inside, but the shape was such that none of it got out.

I have simpler configuration:laser hits half-silvered mirror behind which there is near detector on one optical path and far such antilaser on the second - does turning it on make that detector gets less amount of light ... earlier?

I remember something I read in school about a theroritical 'black hole' so to speak. It was a cavity shaped like an inverted vase coated with a black substance on the inside so that no light shined upon it ever came out. There were infinite reflections on the inside, but the shape was such that none of it got out.

Black body is a theoretical device. It doesn't exist, nor do they claim it should.

Imagine this material in transparent water towers... connected to a steam turbine.

Who needs solar panels, or mirrors?

It only works with coherent light, not the non-coherent variety from the sun. Too many phases going around at once to cancel anything out. You have to do that with white light by using the best black stuff you can make like carbon nanotubes and such, it also absorbs 99.99% of the incoming light and does it a lot cheaper than making a semiconductor device for the job.

I wonder if this could lead to some kind of defense against laser guided weapons? Imagine a jamming device that disrupted a missiles laser guidance. Granted it means capturing the particular laser, which would be difficult, but it makes one wonder.

That's a good thought Kato77. I'm not 100% sure what type of laser the military uses, though, and i'm not sure that it's even common to all the military around the globe. That would add an even bigger level of complexity, figuring out the wavelength then changing the 'antilaser' so that its properties would cancel out the incoming signal. If this could be done it would be years in the future, and by that time will conventional rockets be used? Or will we just be using lasers to burn right through things? My guess is a little of both, but I leave that thought to all of you...

I wonder if this could lead to some kind of defense against laser guided weapons? Imagine a jamming device that disrupted a missiles laser guidance. Granted it means capturing the particular laser, which would be difficult, but it makes one wonder.

The amount of general press coverage this story has received is amazing. I guess the name is really catchy.

I'm not that impressed, though. It's a neat proof of concept but I don't see of what use this could be in an optical computer. It basically turns monochromatic, coherent light into heat and it does this in a very specific setup, with 2 beams head on, as I understand.

Now, if this would work in 90 degrees setup, you could use one beam as a gate for the other one, assuming you can control the phase or wavelength of either of them. That I could see in an optical computer. Then again, several thousands of these switches per square cm dissipating heat might be a problem.